Hemiblocks and the fascicular system: myths and implications

The His bundle divides into the right bundle branch (RBB) and the more complex left bundle branch (LBB). The latter is typically represented as further dividing into anterior and posterior fascicles, and possibly an additional septal fascicle. This construct raises questions as to why conduction can be preserved in patients with “trifascicular block” and why it sometimes very difficult to ablate fascicular or papillary muscle tachycardias. The answer may be that the LBB is much more complex, variable, and interconnected and that the simple familiar constructs may have obstructed our understanding of conduction and tachycardias. Remarkably, the actual explanatory histopathology has been known for nearly half a century, spearheaded by the work of Demoulin and Kulbertus. One picture is worth a thousand words.